The data hiding technique (trademark of IBM) is known as a technique for embedding invisible and inseparable data in still picture data, and moving picture data or audio data. This technique is also called electronic watermarking and has received attention as a technique that can be used to protect copyrights for the contents of multimedia applications. Recently, since digital data have come to be widely employed for moving pictures which have in turn been widely distributed, and since the need for copyright protection has increased for the high value added contents of such applications as movies, the use of data hiding for the control of copying and replaying has been discussed. For embedding additional information in a digital image, generally the information is embedded in a baseband and then JPEG/MPEG compression is performed on the resultant data. Here it should be noted that JPEG is used to represent the Joint Photographic Coding Experts Group, and that MPEG is used to represent the Moving Picture Experts Group.
An electronic watermarking technique for still picture data, moving picture data or audio data is disclosed in, for example, Japanese Patent Application No. Hei 8-159330 (Our docket No. JA9-96-044), Japanese Paten Application No. Hei 8-273551 (Our docket No. JA9-96-076, Japanese Patent Application No. Hei 8-348426 (Our docket No. JA9-96-090), Japanese Patent Application No. Hei 9-088493 (Our docket No. 9-97-156), Japanese Patent Application No. Hei 8-272722 (Our docket No. JA9-97-156), and Japanese Patent Application No. Hei 8-272721 (Our docket No. JA9-96-074). In all the cited prior art, embedding of image data in a baseband is assumed. Even for the embedding of the data contents that are compressed by JPEG/MPEG, generally the data are embedded in the baseband and then the resultant data are compressed, and the data are not directly embedded in a MPEG2 video data stream. This is because the length of the VLC (Variable Length Code) would be changed by directly embedding image data in a compressed MPEG2 video data stream, and thus a content packet can not be coped with.
More specifically, since an AC or DC factor is changed using the VLC, the demultiplexing and the multiplexing of the VLC is required before and after the embedding process, so that the entire length of a bit stream is changed. Therefore, a large buffer is required and reconstruction of the bit stream is difficult. Particularly when DVD data are formed into a packet having a fixed length, the reconstruction of the packet is very difficult. Proposed in Japanese Patent Application No. Hei 8-272721 (Our docket No. JA9-96-074) is a method for directly embedding data in a MPEG2 domain using the forward prediction and backward prediction for a B or P frame. This method also has the same shortcomings in that the length of the VLC is changed and moving picture data having a few B or P frames can not be coped with.
Problems to be Solved by the Invention
It is, therefore, one object of the present invention to provide a method and system for embedding additional information directly in video data.    It is another object of the present invention to provide a method and system for embedding additional information in a packet of video data.    It is an additional object of the present invention to provide a method and system for embedding additional information in MPEG data without changing the length of the VLC.
It is further object of the present invention to provide a method and system for embedding additional information in a video data stream without changing the overall length of the video data stream.
It is still another object of the present invention to provide a method and system for embedding additional information in video data without a large buffer being required. It is a still additional object of the present intention to provide a method and a system, for embedding an electronic watermark in MPEG data, that can cope with a moving picture having a few B or P frames.